PROJECT SUMMARY/ABSTRACT The overarching goal of this research is to develop interventions that improve the overall health and quality of life of individuals post-stroke. It is estimated that 75% of all stroke survivors have comorbid cardiovascular conditions and significantly limited cardiovascular capacity. Many studies have shown that cardiovascular fitness after stroke is not commensurate with independent function and is a major contributor to post-stroke disability. The onset of disability makes it difficult for stroke survivors to engage in physical activity, causing them to remain sedentary and extremely inactive and thus a vicious cycle of disabilityinactivitypoor cardiovascular fitness ensues. Even after intervention, most stroke survivors VO2peak levels remain below the suggested minimum required for independent living. It is therefore necessary to test new interventions that result in greater improvements in cardiovascular fitness and health and walking function after stroke. Among individuals with cardiovascular disease, recent studies have found a high intensity interval training program (HIIT) to be as safe and a more effective stimulus for improving cardiovascular fitness and health compared to traditional aerobic training programs. The efficacy of a HIIT program has not been tested in persons with chronic stroke, but given the success in other populations, it is a promising intervention to investigate in this group. Therefore, the primary aim of this proposal is to test whether a HIIT program results in greater improvements in cardiovascular health and fitness compared to a standard aerobic training program in those with chronic stroke. The secondary aims are to: a) test whether a HIIT program can result in greater improvements in walking function and activity than a standard aerobic training program and b) test whether improvements in cardiovascular fitness and real-world physical activity are linked. Using a randomized controlled experimental design, 53 chronic (> 6 months) stroke survivors, will receive either 12 weeks of a HIIT program or 12 weeks of a standard aerobic treadmill training program. The primary and secondary outcomes of cardiovascular health and fitness and walking activity will be assessed by blinded evaluators prior to initiating treatment (baseline) and immediately after the last treatment (12 weeks post baseline). The expected positive effects of this novel intervention will provide rationale for recommendations for exercise training in chronic stroke and data for larger studies combining aerobic training with interventions designed to improve real-world daily physical activity after stroke.